Photosensitive emulsion for coating on a substrate, such as, for example, a continuous web of paper, cellulose acetate or polyethylene terephthalate, is manufactured in a container. It is drawn from the container through a conduit, usually leading from the bottom of the container, which supplies the emulsion to a coating hopper which applies the emulsion as a uniform thickness layer to the substrate. It is known that in the manufacture of the emulsion in the container, a layer of unusable material, such as froth, usually forms on top of the usable material. The froth is unusable because there must be no air bubbles in the emulsion supplied to the hopper for coating on the substrate. Thus, it is important that when the emulsion is being drawn from the container and supplied through the conduit to the hopper, the unusable material not be supplied to the hopper.
In practice, as the supply of emulsion from one container is approaching depletion, the flow from that container is discontinued but continuity of feed of good emulsion to the hopper is achieved by switching to another similar container containing a new, full batch of emulsion. At present the switching is performed on the basis of the level of material in the container approaching depletion. It is known that with such a method of determining when to switch the supply to the hopper from one container to another, there is considerable waste of usable material in the container being superseded. As is known, some of the materials used in making photographic emulsions are very costly and such waste of usable material represents a very great expense.
In situations when there is not a layer of unusable material lying on top of the usable material in the container, the supply from the first container cannot be totally drawn down through the conduit to be followed by switching to supply from a second container. If such a procedure were allowed there would be the problem of air flowing to the hopper behind the last of the emulsion from the first container and ahead of the beginning of the flow of emulsion from the second container. Such a procedure would allow air into the hopper, which is unacceptable. Thus, in such situations also, for safety's sake, switching between containers has occurred when there is still usable emulsion left in the first container. Such usable emulsion left in the container becomes expensive waste.
It is an object of the present invention to reduce waste.